This invention relates to an exercise machine for strengthening muscles associated with the trunk; and, in particular, to apparatus for applying a variable resistance linearly to the trunk to oppose trunk extension or flexion involving contraction and/or uncontraction of trunk musculature.
The muscles associated with flexion/extension of the trunk (hereafter "trunk muscles") comprise both back and abdominal muscular structures. Posterior trunk extension, for example, involves the erector spinae muscles which, together with its prolongations, fill up the vertebral groove on each side of the spine. Lateral trunk flexion to either the right or left involves the principal abdominal structures (external or descending obliques, internal or ascending obliques, transversalis and abdominal rectus), as well as the latissimus dorsi and other more minor trunk/abdominal structures. The trunk muscles can be exercised by contraction (shortening) or release (lengthening) under load. The term "uncontraction" is used herein to offset the implied exertional volition of "contraction," and indicates a controlled release and muscular assertion during lengthening.
Applicant's copending U.S. patent Application Ser. No. 07/598,131 describes an exercise machine and method for strengthening the abdominals through anterior trunk flexion using a linearly movable plunger directed against the back to apply a variable resistance to the trunk to oppose the flexion. The '131 device includes means to restrain arm and leg movement in order to minimize the involvement of hip flexion. Early experience with linearly applied resistance using the '131 device has been favorable; and, though the '131 device is readily usable for other than anterior flexion, the present application addresses further developments of the linear resistance application method, and addresses exercise machine embodiments particularly suited for posterior trunk extension and lateral trunk flexion.